1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic apparatus and an electrostatic recording apparatus, and more particularly to an image forming apparatus for forming an image on a transferring material being born by a transferring material bearing member.
2. Related Background Art
There is a known image forming apparatus for forming a full color image in which a toner image is formed on a photosensitive drum as a single image bearing member and transferred to a transferring material being born by a transfer drum as a transferring material transporting member. In the apparatus, when toner in a developing unit for forming the toner image decreases, it is necessary to replenish it depending on the decreased amount of toner. For this purpose, there is a known method in which a toner patch is formed on the photosensitive drum by means of the developing unit and the toner in the developing unit is replenished based on the density of the toner patch to keep the toner in the developing unit to be a predetermined amount.
For example, EP-A-519710 discloses that a sensor for detecting the density of a toner patch is provided in a direction of moving a photosensitive drum on the downstream side of a transfer position and on the upstream side of a cleaning position. With this structure, the sensor is prevented from being soiled by toner scattered from the developing unit. Also, EP-A-519710 discloses that when the toner patch passes the transfer position such that it is not conveyed to the surface of a transfer drum, the pressing of a pressing member for pressing a transferring material bearing sheet of a transfer drum toward the photosensitive drum is released.
Further, there is an image forming apparatus in which in order to form a full color image at high speed, a plurality of photoreceptors for respective colors, i.e., four photoreceptors for respective cyan, magenta, yellow and black are provided and toner images of respective colors are transferred to a transferring material being born by a transfer belt as a transferring material bearing member. In this apparatus, for example, when detecting a toner patch formed on the photoreceptor for black toner in the same manner as in EP-A-519710, the following problem occurs even though the transfer belt is detached from the photoreceptors while the toner patch is passing the transfer position.
That is, when the pressing of pressing members for pressing the transfer belt is released, the transfer belt is vibrated due to the release operations of the pressing members in the respective magenta, cyan and yellow stations and the vibration reaches the black station located downstream as the transfer belt moves. As a result, in the downstream black station, the toner patch is brought into contact with the vibrating transfer belt, so that the toner patch cannot be detected accurately.